Heat exchanger leak testing method and apparatus

ABSTRACT

A method for detecting the presence of a leak between two fluid paths of an assembled plate heat exchanger by evacuating one or both fluid paths to a low pressure, introducing a test gas to one of the fluid paths to create a pressure differential between the fluid paths, and testing for the presence of test gas in the second fluid path.

FIELD OF THE INVENTION

This invention generally relates to heat exchangers for fluids. Inparticular, the invention relates to a method and apparatus for leaktesting a heat exchanger, especially, but not exclusively, a plate heatexchanger.

BACKGROUND OF THE INVENTION

Heat exchangers typically comprise separate flow paths for two fluidsarranged in close contact to facilitate heat transfer between the fluidsthereby cooling the higher temperature fluid while simultaneouslyheating the lower temperature fluid as they flow through the heatexchanger. In plate heat exchangers the flow paths are commonly providedby a series of metallic plates packed vertically in sequence withgaskets between the plates to seal the plates and separate the flowpaths. This arrangement allows complex flow paths of long length foroptimum heat transfer to be achieved.

Plate heat exchangers are commonly used for food processing where one ofthe fluids is a food product to be heated or cooled by heat exchangewith the other fluid. To maintain quality of the food product beingprocessed, it is important to avoid the food product being contaminatedby cross-flow between the flow paths within the heat exchanger. To thisend it is routine practice to check plate heat exchangers for any faultsthat may allow cross-flow to occur. Assembly/disassembly of plate heatexchangers is time consuming and it is desirable to be able to carry outsuch checks on the assembled heat exchanger so that disassembly forrepair is only required when a leak is detected.

To this end, various methods have been proposed in the prior art forleak testing an assembled plate heat exchanger. However, these methodsare generally not satisfactory for detecting small faults such asfatigue cracks or corrosion pinholes that can occur in the heatexchanger plates. While such faults may only allow a very small amountof cross-flow to occur, the resulting contamination of the food productis such that it must be thrown away and the heat exchanger disassembledto find and repair the fault. This is wasteful of the food product andtime consuming to carry out a repair, all of which adds to cost.

SUMMARY

The present invention has been made from a consideration of theforegoing and seeks to provide a method and apparatus for leak testing aheat exchanger where these problems are at least mitigated.

Thus, it is a desired aim of the present invention to provide a methodand apparatus for leak testing a heat exchanger, especially a plate heatexchanger, that is capable of detecting smaller faults in the heatexchanging portions of the heat exchanger than hitherto possible withoutdisassembly of the heat exchanger.

From one aspect, the invention generally provides a method for detectingthe presence of a leak between two fluid paths of a heat exchanger. Inone step of the method, at least one of the fluid paths may be evacuatedto a low pressure. In another step of the method, a test gas may beintroduced to one of the fluid paths to create a pressure differentialbetween the fluid paths. In another step of the method, a test for thepresence of test gas in the second fluid path may be carried out.

In one preferred embodiment, the method comprises

sealing a test section including first and second fluid paths;

evacuating said first and second fluid paths;

introducing a test gas to said first fluid path so that a pressuredifferential is created between said first fluid path and said secondfluid path; and

testing for the presence of said test gas in said second fluid pathwhereby the presence of a leak between said first and second fluid pathsof said test section may be determined.

Preferably, the first and second fluid paths are evacuated tosubstantially the same pressure prior to introducing the test gas to thefirst fluid path. Typically, the evacuated pressure is below atmosphericpressure and is preferably below 0.5 bar and more preferably is about0.1 bar or less.

Preferably, the second fluid paths is evacuated at or near to one end ofthe test section and the test gas is introduced into the first fluidpath at or near to the other end of the test section. Typically, testgas is introduced to the first section to a pressure above atmosphericpressure, preferably at least 2 bar and more preferably about 4 bar.

Preferably, the presence of the test gas in the second fluid path isdetected by testing fluid evacuated from the second fluid path. Forexample, fluid evacuated from the second fluid path after theintroduction of test gas to the first fluid path may be collected in avessel provided with a detector for testing for the presence of the testgas.

In some embodiments, fluid evacuated from the second fluid path fortesting includes a tracer gas introduced to the second fluid path afterthe introduction of test gas to the first fluid path. Preferably, thetracer gas flows through substantially the entire second fluid pathwhereby test gas that leaks from the first fluid path to the secondfluid path at any point is picked up and evacuated from the second fluidpath with the tracer gas.

The tracer gas may be introduced to the second fluid path in one or moreseparate volumes. Alternatively, a controlled flow of the tracer gas maybe provided and detection of test gas in the tracer gas evacuated fromthe second fluid path used to locate the position of a leak between thefirst and second fluid paths

Preferably, the test gas is hydrogen which may be mixed with an inertgas such as nitrogen to reduce the risk of an explosion. Where providedthe tracer gas may be air.

The method may one or more of cleaning, draining and drying the testsection prior to commencing the test procedure. The method hasparticular application to detecting leaks in assembled plate heatexchangers. For the sake of completeness, it should be added that theleaks to be discovered are preferably those lying in the heat exchangingportions of the plate heat exchanger. It will be readily recognised thatthe two fluid paths preferably overlap side by side in the heat exchangezone only.

From another aspect, the invention generally provides apparatus fordetecting the presence of a leak between two fluid paths of a heatexchanger. The apparatus may be provided with means for evacuating atleast one of the fluid paths to a low pressure. The apparatus may beprovided with means for introducing a test gas to one of the fluid pathsto create a pressure differential between the fluid paths. The apparatusmay be provided with means for testing for the presence of test gas inthe second fluid path.

In a preferred embodiment, the apparatus comprises:

means for evacuating a sealed test section including said first andsecond fluid paths;

means for introducing a test gas to said first fluid path such that thefirst fluid path is at a higher pressure than the second fluid pathwhereby a pressure differential is created between said first and secondfluid paths; and

means for testing for the presence of test gas in the second fluid path.

Preferably, the evacuation means comprises a vacuum pump connected tothe first and second fluid paths, and valve means is provided forselectively isolating each of the first and second fluid paths from theevacuation means.

Preferably, the gas introduction means comprises a gas cylinderconnected to the first fluid path, and valve means is provided forisolating the gas introduction means from the first fluid path.

Preferably, the testing means comprises a detector for detecting thepresence of the test gas in fluid evacuated from the second fluid path.

In some embodiments, means is provided for introducing a tracer gas tothe second fluid path and valve means is provided for isolating thetracer gas introduction means from the second fluid path.

Preferably, tracer gas introduction means provides a controlled flow oftracer gas and the amount of test gas in the tracer gas is monitoredover time for determining an approximate location of a leak in the testsection.

The invention will now be described in more detail by way of exampleonly with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a schematic layout of system for leak testing aplate heat exchanger embodying the method and apparatus of the presentinvention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the following description, the construction of the plate heatexchanger is not shown and described in detail as such heat exchangersare familiar to those skilled in the art and do not form an essentialpart of the invention which, as will be apparent from the followingdescription, has application to all types and constructions of plateheat exchangers as well as other types of heat exchangers where separateflow paths for two fluids are arranged in close contact to facilitateheat transfer between the fluids.

Referring to the drawing, the two flow paths of a plate heat exchanger 2are depicted as a donor side 4 and a receiving side 6 and are shown asbeing in close contact within a heat transfer section 8 of the heatexchanger. The terms “donor side” and “receiving side” are usedhereinafter purely to distinguish between the flow paths for the purposeof clarity and are not descriptive of the heat exchanger 2 and aretherefore not limiting on the scope of the invention.

The system for leak testing the plate heat exchanger 2 includes a source10 of a test gas such as, but not limited to, hydrogen (typically 5%hydrogen and 95% nitrogen or other inert gas although other ratios maybe employed), a vacuum pump 12 and for at least some modifications anelastic vessel 14 containing a tracer gas (typically, but not limitedto, air). The hydrogen source 10 is connected to the donor side 4 by aline 16 provided with valves 18 and 22 between which is fitted a gaspressure regulator 20 with outlet pressure gauge 24. The vacuum pump 12is connected to the receiving side 6 by a line 34 containing valve 28.The donor side 4 is connected to the receiver side 6 using line 26fitted with valves 30 and 32. Between valves 30 and 32 is fitted a quickdisconnect 40. Line 26 connects to line 34 at a point between valve 28and the vacuum pump 12.

Prior to the test procedure, the test section is normally thoroughlycleaned in place (CIP) using existing CIP procedures adopted by the enduser to normally clean the equipment. It is then flushed with cleanwater to remove trace wash chemicals. However, such cleaning is notessential to the invention and may be omitted. Next, if free drainage ofthe test section is not possible, then forced drainage (blast out) usinga blast of gas such as compressed air or nitrogen is employed.Connection of air lines can be made to valves 35 and 37 with exhaustfrom valves 41 and 42. This blow down may be repeated several times forboth sides using different pressure or gas release rates until bothsides are sufficiently drained to ensure that the heat transfer area isnot covered by a standing water level. The test section is now ready forthe test procedure. In a modification, a hot gas such as hot air may becirculated through one or both sides 4,6 of the test section beforestarting the test procedure. The hot gas may help to dry liquid held bycapillary action in any faults in the plates separating the donor side 4and receiving side 6 after the initial cleaning and/or draining stages.

The first step of the test procedure is to seal the test section byclosing valves 35,37,41 and 42. The vacuum pump 12 is then started andvalves 28,30,32 opened to connect the pump 12 to the donor side 4 andreceiving side 6. The vacuum pump 12 is operated to evacuate the donorside 4 and receiving side 6 to a low pressure, typically about 0.1 bar(10 kPa) or lower, as indicated by vacuum gauges 38 and 39 connected tothe receiving and donor side respectively. Valves 28,30 and 32 areclosed and the vacuum pump 12 stopped. The evacuation to low pressurehelps to evaporate and remove liquid that may be held by capillaryforces in any fault in the plates separating the donor side 4 andreceiving side 6 thereby improving sensitivity of the test procedure todetect such faults.

After a period of time, say 10 minutes, the readings on the vacuumgauges 38 and 39 are checked to ensure the system is sealed. If thepressure has increased, the system seal is checked and the step repeateduntil the low pressure is maintained. The next step of the testprocedure is to positively disconnect the donor side 4 and receivingside 6 by means of a quick connection coupling 40 situated between thevalves 30,32. Use of the disconnect ensures that the donor and receiversides are positively isolated from each other rather than just relyingon the integrity of valves 30 and 32 for positive isolation. Valve 43 isnow closed to isolate vacuum gauge 39. Valve 18 is opened to connect thehydrogen source to the regulator 20 and the pressure as measured on theregulator outlet increased until it reads approximately 4 bar (400 KPa)or higher then valve 22 is opened to charge the donor side 4. Aftercharging is complete valves 22 and 18 are closed.

Since the donor side 4 was evacuated to low pressure before pressurisingwith the test gas, the test gas can flow more easily throughout all ofthe donor side 4 without being diluted by air present in the donor sidebefore evacuation. The donor side 4 is now at a significantly higherpressure than the receiving side 6 and the test gas can flow readilyfrom the donor side 4 to the receiving side 6 only through any faultspresent in the heat transfer area of the plates separating the donorside 4 and receiving side 6. After a period of time, say 15 minutes anempty elastic vessel 44 such as a plastic bag is connected to the vacuumpump outlet via a valve 46 and quick disconnect 48 The vacuum pump 12 isstarted and valve 28 opened to begin gas collection from the receiverside into the elastic vessel 44. On filling vessel 44 can be removed viathe disconnect 48 and the gas tested with a suitable detector (notshown). The presence of hydrogen in the collected gas indicates thepresence of a fault in the plates separating the donor side 4 and thereceiving side 6. In a modification to the above procedure anotherelastic vessel 14 is filled with a tracer gas (typically air) andconnected to valve 36 before any gas collection into vessel 44commences. During gas collection into vessel 44, valve 36 is opened andthe tracer gas passed into the receiving side 6. The tracer gas spreadsthroughout the receiving section and is enriched with any hydrogen thathas passed from the donor side 4 to the receiving side 6. The evacuatedgas (tracer gas plus any leaked hydrogen) is again collected in thevessel 44 and tested to determine the presence of a leak between thedonor side 4 and the receiving side 6.

The introduction of the known volume of tracer gas in the vessel 14 tothe receiving side 6 assists detection of a small fault in a large,complex heat exchanger where the test section may have long, tortuousflow paths that make recovery and detection of very small volumes ofhydrogen leaking from the donor side 4 to the receiving side 6 moredifficult and problematic. Thus, the tracer gas raises the pressure inthe receiving side 6 slightly, although the pressure is stillsubstantially below the pressure of the test gas in the donor side 4,and, because the receiving side 4 is still at low pressure, the tracergas can spread rapidly throughout the receiving side 6 so that anyhydrogen present in the receiving side 6 can be more completely removedalong with the tracer gas and detected when the receiving side 6 isevacuated. This may be further enhanced by arranging for the receivingsection 6 to be evacuated at one end and the tracer gas to be introducedat the other end so that the tracer gas flows through the entire sectionunder test.

In a further modification, the tracer gas is introduced more thanonce—multiple shots—and the receiving section 6 evacuated after eachintroduction of tracer gas to ensure that any hydrogen leaking from thedonor side 4 to the receiving side 6 from a fault in the platesseparating the donor side 4 and receiving side 6 anywhere in the testsection has been found.

In a further modification, the tracer gas is introduced to the receivingsection 6 at or near to one end of the test section as a steady flowwhile evacuating the receiving section 6 at or near to the other end ofthe test section. In this way, it is possible to approximately correlatethe position of the fault in the plate from a study of the hydrogenconcentration in the tracer gas versus time at the detecting end of thetest section.

The above-described method in which both sides of the test section—thedonor side 4 and receiving side 6—are evacuated prior to introduction ofthe test gas to the donor side 4 and, where employed, tracer gas to thereceiving side 6 is improves the detection of test gas that has leakedfrom the donor side 4 into the receiving side 6. As a result, the methodcan detect faults where only a small volume of test gas leaks into thereceiving section making it particularly suitable for heat exchangershaving long, complex, tortuous flow paths where small volumes of leakedtest gas may be less easy to collect and detect. However, for heatexchangers where the flow paths are of simpler construction withshorter, less complex and tortuous flow paths, it may not be necessaryto evacuate both sides of the test section in order to collect anddetect test gas that has leaked into the receiving section through afault in the heat transfer area of the test section. Thus, the inventionincludes a method in which only one side of the test section, forexample the receiving side, is evacuated before introducing the test gasat an elevated pressure to create a pressure differential between thedonor side and receiving side such that test gas flows into thereceiving side through any fault in the heat transfer area of the testsection.

A plate heat exchanger may comprise one or more sections and where morethan one section, each section may be tested separately in turn forleaks or two or more or all sections may be linked and testedsimultaneously for leaks. Whichever option is employed may depend on thearrangement and complexity of the flow paths in each section.

As will be appreciated, the invention provides a method and apparatusfor testing a plate heat exchanger for leaks while assembled thatenables even small faults such as hairline cracks or pinholes to bedetected in a reliable manner. In its broadest application, the testprovides an indication of the presence/absence of a leak so that onlyheat exchangers found to have a leak need to be disassembled for repairthereby reducing or avoiding unnecessary disassembly. In one preferredembodiment, the position of a leak within the heat exchanger may beapproximately determined so that the plate with the fault can be moreeasily located on disassembly of the heat exchanger.

The invention has particular application to leak testing plate heatexchangers of the type in which flow paths for two heat exchange fluidsare provided by a package of plates, typically metal or other materialhaving a high thermal conductivity for heat transfer, arranged insequence with gaskets between the plates to seal the plates and separatethe flow paths such that the flow paths extend side by side rather thanone inside the other and at least partially overlap side by side in theheat exchanging zones of the surfaces. While the invention has beendescribed with particular application to detecting a leak in a plateheat exchanger, it will be understood that the invention has widerapplication to other types and constructions of heat exchanger havingflow paths arranged in close contact for heat exchange between fluidsflowing through the flow paths.

It will also be understood that while the invention has been describedwith reference to embodiments illustrating the best method and apparatuscurrently known to the inventors, variations and modifications to themethod and apparatus may be made without departing from the principlesor concepts described herein. For example, the test gas and, whereprovided, tracer gas, may comprise any suitable gas or mixture of gasesthat enable the presence of test gas in the receiving section to bereadily detected. The donor side and receiving side may be evacuated toaround 0.1 bar or described or any other suitable low pressure while thedonor side may be charged with test gas at around 4 bar or any othersuitable high pressure. As used herein “low pressure” and “highpressure” are used to indicate the existence of a pressure differentialsufficient for test gas to flow from the donor side to the receivingside through any faults in the plates separating the donor side andreceiving side and the actual pressures may vary from those givenwithout altering the operation of the invention.

1. A method for detecting a leak between separate first and second fluidpaths of a heat exchanger arranged in close heat exchange relationship,the method comprising: sealing a test section including said first andsecond fluid paths; evacuating said first and second fluid paths;introducing a test gas to said first fluid path so that a pressuredifferential is created between said first fluid path and said secondfluid path; and testing for the presence of said test gas in said secondfluid path whereby the presence of a leak between said first and secondfluid paths of said test section may be determined.
 2. A methodaccording to claim 1 wherein, said first and second fluid paths areevacuated to substantially the same pressure prior to introducing saidtest gas to said first fluid path.
 3. A method according to claim 2wherein, said first and second fluid paths are evacuated to a pressureof about 0.1 bar or less.
 4. A method according to claim 1 wherein, saidsecond fluid path is evacuated at or near to one end of said testsection and said test gas is introduced at or near to the other end ofsaid test section within said first fluid path.
 5. A method according toclaim 4 wherein, said test gas is introduced to said first section to apressure of about 4 bar.
 6. A method according to claim 1 wherein, afterintroducing said test gas to said first fluid path, the presence of saidtest gas in said second fluid path is detected by testing fluidevacuated from the second fluid path.
 7. A method according to claim 4wherein, prior to testing for the presence of test gas in said secondfluid path, a tracer gas is introduced to said second fluid path at ornear to the other end of said test section and evacuated from saidsecond fluid path at or near to said one end of the test section.
 8. Amethod according to claim 7 wherein, said tracer gas is introduced tosaid second fluid path and the presence of said test gas in said secondfluid path is detected by collecting and testing tracer gas evacuatedfrom said second fluid path.
 9. A method according to claim 8 wherein,the step of introducing said tracer gas to said second fluid path isrepeated.
 10. A method according to claim 7 wherein, a controlled flowof said tracer gas is introduced to said second fluid path and detectionof test gas in tracer gas evacuated from said one end of the secondfluid path used to locate the position of a leak between said first andsecond fluid paths
 11. A method according to claim 1 wherein, said testgas is hydrogen.
 12. A method according to claim 11 wherein, said testgas comprises a mixture of hydrogen and an inert gas.
 13. A methodaccording to claim 1 further including prior to sealing said testsection one or more of cleaning, draining and drying said test section.14. A method according to claim 1 wherein said heat exchanger is a plateheat exchanger.
 15. Apparatus for detecting a leak between separatefirst and second fluid paths of a heat exchanger arranged in close heatexchange relationship, the apparatus comprising: means for evacuating asealed test section including said first and second fluid paths; meansfor introducing a test gas to said first fluid path such that the firstfluid path is at a higher pressure than the second fluid path whereby apressure differential is created between said first and second fluidpaths; and means for testing for the presence of test gas in the secondfluid path.
 16. Apparatus according to claim 15 wherein, said evacuationmeans comprises a vacuum pump connected to said first and second fluidpaths and valve means is provided for selectively isolating each of saidfirst and second fluid paths from said evacuation means.
 17. Apparatusaccording to claim 15 wherein, said gas introduction means comprises agas cylinder connected to said first fluid path and valve means isprovided for isolating said gas introduction means from said first fluidpath.
 18. Apparatus according to claim 15 wherein, said testing meanscomprises a detector for detecting the presence of said test gas influid evacuated from said second test section.
 19. Apparatus accordingto claim 15 including means for introducing a tracer gas to said secondfluid path and valve means is provided for isolating said tracer gasintroduction means from said second fluid path.
 20. Apparatus accordingto claim 19 including means for providing a controlled flow of tracergas and monitoring the amount of test gas in the tracer gas fordetermining an approximate location of a leak in the test section.
 21. Amethod for detecting a leak between first and second fluid paths of aheat exchanger arranged side-by-side in close heat exchangerelationship, the method comprising: sealing a test section includingsaid first and second fluid paths; evacuating at least one of said firstand second fluid paths; introducing a test gas to said first fluid pathso that a pressure differential is created between said first fluid pathand said second fluid path; and testing for the presence of said testgas in said second fluid path whereby the presence of a leak betweensaid first and second fluid paths of said test section may bedetermined.
 22. A method according to claim 21 wherein the second fluidpath is evacuated.
 23. A method according to claim 21 wherein the firstand second fluid paths are evacuated.
 24. A method according to claim 21wherein the heat exchanger is a plate heat exchanger comprising apackage of plates arranged in sequence with gaskets between the platesto seal the package and separate the flow paths.
 25. Apparatus fordetecting a leak between separate first and second fluid paths of a heatexchanger arranged side-by-side in close heat exchange relationship, theapparatus comprising: means for sealing a test section including saidfirst and second fluid paths; means for evacuating at least one of saidfirst and second fluid paths; means for introducing a test gas to saidfirst fluid path such that the first fluid path is at a higher pressurethan the second fluid path whereby a pressure differential is createdbetween said first and second fluid paths; and means for testing for thepresence of test gas in the second fluid path.